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Cell Division (Stem cells (Therapeutic cloning (Transplanting stem cells -…
Cell Division
Stem cells
Stem cells are cells that have not undergone differentiation. A cell which has not yet become specialised is called undifferentiated.
Embryonic stem cells
Form when an egg and sperm cell fuse to form a zygote
They can differentiate into any type of cell in the body
An embryo develops from a fertilised egg. Cells at the early stages in the development of the embryo are stem cells.
If cells are removed from the embryo – called embryonic stem cells - they will differentiate into any cell type.
Adult stem cells
Some stem cells remain in the bodies of adults – adult stem cells. Adult stem cells are found in limited numbers at certain locations in the body.
Adult stem cells can differentiate into related cell types only, for example, bone marrow cells can differentiate into blood cells and cells of the immune system but not other cell types.
Adult stem cells can be found in several regions of the body, including the: brain, eyes, blood, heart, liver, bone marrow, skin, muscle
Meristems in plants
Cell division in plants occurs in regions called meristems.
(In a growing shoot, new cells are being produced continuously near the tip. As the cells become older, further away from the tip, they become differentiated – they enlarge and develop vacuoles.)
● Found in root and shoot tips
● They can differentiate into any type of plant, and have this ability throughout the life of the plant
● They can be used to make clones of the plant- this may be necessary if the parent plant has certain desirable features (such as disease resistance), for research or to save a rare plant from extinction
Therapeutic cloning
Transplanting stem cells - Adult stem cell transplants use a patient's own stem cells. They are therefore genetically identical and will not be rejected by the patient's immune system.
Embryonic stem cells will always come from a donor – unless stem cells were collected from the patient as an embryo.
Therapeutic cloning could produce stem cells with the same genetic make-up as the patient.
Therapeutic cloning involves an embryo being produced with the same genes as the patient.
The embryo produced could then be harvested to obtain the embryonic stem cells. The technique involves the transfer of the nucleus from a cell of the patient, to an egg cell whose nucleus has been removed.
Clinical issues:
There is no guarantee of how successful these therapies will be, for example, the use of stem cells in replacing nerve cells lost in Parkinson’s disease patients.
The current difficulty in finding suitable stem cell donors.
The difficulty in obtaining and storing a patient’s embryonic stem cells. These would have to be collected before birth - some clinics offer to store blood from the umbilical cord when a person is born.
Mutations have been observed in stem cells cultured for a number of generations, and some mutated stem cells have been observed to behave like cancer cells.
Cultured stem cells could be contaminated with viruses that would be transferred to a patient.
Social issues:
Educating the public about what stem cells can, and can't do, is important.
Whether the benefits of stem cell use outweigh the objections.
Much of the research is being carried out by commercial clinics, so reported successes are not subject to peer review. Patients could be exploited by paying for expensive treatments and being given false hope of a cure as stem cell therapies are only in their developmental stages.
Ethical issues:
A source of embryonic stem cells is unused embryos produced by in vitro fertilisation (IVF).
For therapeutic cloning is it right to create embryos for therapy, and destroy them in the process?.
Embryos could come to be viewed as a commodity, and not as an embryo that could develop into a person.
At what stage of its development should an embryo be regarded as, and treated as a person?
Using human stem cells
Stem cells could be used to replace cells that have been damaged or destroyed, eg: in type 1 diabetes, in cases of multiple sclerosis, which can lead to paralysis, in cases of spinal cord or brain injury, that have led to paralysis.
Binary Fission
1.We see the intact bacterial chromosome (which is circular). It has two regions called the origin of replication and the terminus of replication, which are located diametrically opposite to one another on the chromosome.
2.The chromosome opens at the origin of replication, and the two DNA strands are copied, with replication proceeding in opposite directions on the two strands.
3.Copying continues, and the cell elongates. The new origins of replication move apart, towards opposite ends of the cell.
4.A septum (wall) forms down the middle of the cell, partitioning it into two new cells, each with one of the two (now-complete) bacterial chromosome copies.
5.The cell pinches in two. We now have two new bacteria!
Chromosomes
The structure made of DNA that codes for all the characteristics of an organism.
Each human body cell contains 46 chromosomes. These can be arranged into 23 pairs.
Carry genetic information in a molecule called DNA.
Each chromosome in a pair carries the same types of genes. The 23rd pair are the sex chromosomes
Mitosis and the cell cycle
Cells divide when:
an organism grows
an organism becomes damaged and needs to produce new cells
It is essential that any new cells produced contain genetic information that is identical to the parent cell.
Cell cycle- The series of stages that a cell goes through as it is growing and dividing
During interphase, the cell grows and makes a copy of its DNA.
During the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells.
Interphase
G_1 phase
During the G_1 phase, also called the first gap phase, the cell grows physically larger, copies organelles (such as mitochondria and ribosomes), and makes the molecular building blocks it will need in later steps.
S phase
In the S phase, the cell synthesizes a complete copy of the DNA in its nucleus. The single strand of DNA that makes up each chromosome becomes double-stranded(becomes X shaped). It also duplicates a microtubule-organizing structure called the centrosome. The centrosomes help separate DNA during the M phase.
G_2 phase
During the second gap phase, or G_2 phase, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G_2 phase ends when mitosis begins.
Mitotic phase
Mitosis
Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to itself.
During development and growth, mitosis populates an organism’s body with cells, and throughout an organism’s life, it replaces old, worn-out cells with new ones. For single-celled eukaryotes like yeast, mitotic divisions are actually a form of reproduction, adding new individuals to the population.
1.The mitotic spindle starts to form, the chromosomes starts to condense,and the nucleolus starts to disappears.
The nuclear envelope breaks down and the chromosomes are fully condensed.
2.Chromosomes line up at the metaphase plate, under tension from the mitotic spindle. The two sister chromatids of each chromosomes are captured by microtubules from opposite spindle poles.
3.The sister chromatids separate from one another and are pulled towards opposite poles of the cell. The microtubules that are not attached to chromosomes push the two poles of the spindle apart, while the kinetochore microtubules pull the chromosomes towards the poles.
4.The spindle disappears, a nuclear membrane re-forms around each set of chromosomes, and a nucleolus reappears in each new nucleus. The chromosomes also start to decondense.
5.When division is complete, it produces two daughter cells. Each daughter cell has a complete set of chromosomes, identical to that of its sister (and that of the mother cell). The daughter cells enter the cell cycle.
Cytokinesis
The cytoplasm of the cell is split into two, making two new cells. Cytokinesis usually begins just as mitosis is ending, with a little overlap. (It happens differently in animal cells and plant cells)